The present invention relates to an antenna device having a single antenna which bifunctions as an antenna of a radio receiver for AM and FM bands and an antenna of a mobile telephone transceiver. More particularly, the present invention is concerned with an antenna device having a collinear array antenna in the form of half-wave dipole antennas which are adapted for a mobile telephone receiver and are stacked in two stages. This kind of antenna device is applicable to an AM-FM radio receiver and a mobile telephone transceiver both of which are mounted on a vehicle, allowing the receiver to efficiently receive AM and FM broadcasts and allowing the transceiver to transmit and receive a mobile telephone signal.
It has been customary to provide a vehicle-mounted AM-FM radio receiver with an about 1.0 to 1.4 meter long rod antenna which is telescopically mounted on, for example, the roof of a vehicle. A 75 or 50 ohm coaxial cable is connected to the base end of the rod antenna. AM and FM broadcast signals are delivered by the coaxial cable to the AM-FM radio receiver which is mounted on the console inside of the vehicle independently of the antenna. A vehicle-mounted or car telephone which is extensively used today has a two-stage collinear array antenna. This kind of antenna is constituted by stacking half-wave dipole antennas adapted for a mobile telephone signal one above the other and is mounted on, for example, the hood of a vehicle. A 75 or 50 ohm coaxial cable connects the base end of the collinear array antenna to a telephone transceiver which is located in the vehicle cabin, allowing the tranceiver to transmit and receive a telephone signal.
As stated above, a vehicle loaded with an AM-FM radio receiver and a telephone transceiver customarily has both of a rod antenna and a two-stage collinear array antenna. Mounting a plurality of antennas on a vehicle is undesirable because they mar the appearance of an otherwise fine exterior design of the vehicle and because they aggravate the hissing sound while the vehicle is operated. Of course, the plurality of antennas protruding to the outside from the vehicle body are more dangerous than a single antenna. To reduce the number of antennas, the rod antenna for the AM-FM radio and which is about 1 meter long may be so constructed as to play the role of the antenna for the telephone transceiver also. It is to be noted that the length of about 1 meter of the rod antenna corresponds to substantially one-quarter wavelengh of the FM broadcast signal and is selected to achieve a high antenna gain for FM broadscasts by antenna resonance.
The antenna protruding from the vehicle body should be as short as possible as mentioned above in order to provide the vehicle with attractive appearance, to reduce hissing sound, and to prevent the antenna from hitting against or contacting a garage, structures on the road, etc. Another approach is, therefore, to use the two-stage colliear array antenna for the telephone transceiver as the antenna of the AM-FM radio receiver also. However, this antenna is only 40 centimeters long or so and cannot cause the FM broadcast signal to resonate, failing to function as an FM antenna due to the critically low gain. Such an antenna is not desirable for receiving AM broadcasts either, because the shorter the length, the lower the signal strength which can be received is. It is impractical, therefore, to allow the collinear array antenna having the convenional structure to be shared by the telephone transceiver and the AM-FM receiver.
The two-stage collinear array antenna and the coaxial cable have to be matched so that the telephone signal coming in through the antenna may be efficiently delivered to the telephone transceiver and the telephone signal may be efficiently radiated from the antenna. For this purpose, there has been proposed a feed structure in which the base end of the collinear array antenna is extended to the inside of the vehicle and, at a position where the antenna can be matched to the coaxial cable, electrically connected to the coaxial cable. However, when the carrier frequency is as high as 870 to 940 MHz as with the telephone signal, the wavelength is correspondingly short. Hence, when the position where the antenna is electrically connected to the coaxial cable is deviated even slightly, the deviation is critical when it comes to the wavelength and prevents desired matching from being achieved. It is desirable, therefore, that the position of the feed point be adjustable at the production stage.
U.S. Pat. No. 4,847,629 discloses an arrangement wherein the role of the antenna for AM-FM radio reception and that of the antenna for mobile telephone reception are played by a single antenna. With this arrangement, it is also possible to adjust the position of the feed point defined at the base end of the single antenna for thereby adjusting the input/output impedance. The drawback with such an implementation, however, is that a stub or a balun having a length approximately that of one-quarter wavelength protrudes noticeably into the vehicle cabin from the feed point. Moreover, no consideration is given to the decrease in the strength of the received FM broadcast signal which is ascribable to the short antenna.